I won't argue too hard one way or the other. I avoided medicated feed, and many people use it with no apparent difficulty.I've heard that your not supposed to give them medicated, because they could get over medicated somehow..
There is a logic to avoiding anything medicated if the medication is not called for. Also, though I don't fully understand it, I have read that amprolium - the medication that is put in chick starter (I believe that's it) - blocks thiamine uptake in the coccidia. My question is, does it block thiamine uptake in the duckling? I have not read any peer-reviewed material on that, so it remains a question for me.
If the only thing one can get is medicated - well - that's a choice to make. It is much better, I feel, than trying to feed corn or similar, as there is so little nutrient-wise in corn.
Part of the reasoning, also, about ducklings and medicated feed is that they consume their food so much faster than chicks do, so they are getting more of the medication per day.